Apache HTTP Server

Content Negotiation

Apache's support for content negotiation has been updated to
meet the HTTP/1.1 specification. It can choose the best
representation of a resource based on the browser-supplied
preferences for media type, languages, character set and
encoding. It is also implements a couple of features to give
more intelligent handling of requests from browsers which send
incomplete negotiation information.

Content negotiation is provided by the mod_negotiation module,
which is compiled in by default.

About Content Negotiation

A resource may be available in several different
representations. For example, it might be available in
different languages or different media types, or a combination.
One way of selecting the most appropriate choice is to give the
user an index page, and let them select. However it is often
possible for the server to choose automatically. This works
because browsers can send as part of each request information
about what representations they prefer. For example, a browser
could indicate that it would like to see information in French,
if possible, else English will do. Browsers indicate their
preferences by headers in the request. To request only French
representations, the browser would send

Accept-Language: fr

Note that this preference will only be applied when there is
a choice of representations and they vary by language.

As an example of a more complex request, this browser has
been configured to accept French and English, but prefer
French, and to accept various media types, preferring HTML over
plain text or other text types, and preferring GIF or JPEG over
other media types, but also allowing any other media type as a
last resort:

Apache 1.2 supports 'server driven' content negotiation, as
defined in the HTTP/1.1 specification. It fully supports the
Accept, Accept-Language, Accept-Charset and Accept-Encoding
request headers. Apache 1.3.4 also supports 'transparent'
content negotiation, which is an experimental negotiation
protocol defined in RFC 2295 and RFC 2296. It does not offer
support for 'feature negotiation' as defined in these RFCs.

A resource is a conceptual entity
identified by a URI (RFC 2396). An HTTP server like Apache
provides access to representations of the
resource(s) within its namespace, with each representation in
the form of a sequence of bytes with a defined media type,
character set, encoding, etc. Each resource may be associated
with zero, one, or more than one representation at any given
time. If multiple representations are available, the resource
is referred to as negotiable and each of its
representations is termed a variant. The ways
in which the variants for a negotiable resource vary are called
the dimensions of negotiation.

Negotiation in Apache

In order to negotiate a resource, the server needs to be
given information about each of the variants. This is done in
one of two ways:

Using a type map (i.e., a *.var
file) which names the files containing the variants
explicitly, or

Using a 'MultiViews' search, where the server does an
implicit filename pattern match and chooses from among the
results.

Using a type-map file

A type map is a document which is associated with the
handler named type-map (or, for
backwards-compatibility with older Apache configurations, the
mime type application/x-type-map). Note that to
use this feature, you must have a handler set in the
configuration that defines a file suffix as
type-map; this is best done with a

AddHandler type-map .var

in the server configuration file. See the comments in the
sample config file for more details.

Type map files have an entry for each available variant;
these entries consist of contiguous HTTP-format header lines.
Entries for different variants are separated by blank lines.
Blank lines are illegal within an entry. It is conventional to
begin a map file with an entry for the combined entity as a
whole (although this is not required, and if present will be
ignored). An example map file is:

qs values can vary in the range 0.000 to 1.000. Note that
any variant with a qs value of 0.000 will never be chosen.
Variants with no 'qs' parameter value are given a qs factor of
1.0. The qs parameter indicates the relative 'quality' of this
variant compared to the other available variants, independent
of the client's capabilities. For example, a jpeg file is
usually of higher source quality than an ascii file if it is
attempting to represent a photograph. However, if the resource
being represented is an original ascii art, then an ascii
representation would have a higher source quality than a jpeg
representation. A qs value is therefore specific to a given
variant depending on the nature of the resource it
represents.

The full list of headers recognized is:

URI:

uri of the file containing the variant (of the given
media type, encoded with the given content encoding). These
are interpreted as URLs relative to the map file; they must
be on the same server (!), and they must refer to files to
which the client would be granted access if they were to be
requested directly.

Content-Type:

media type --- charset, level and "qs" parameters may be
given. These are often referred to as MIME types; typical
media types are image/gif,
text/plain, or
text/html; level=3.

Content-Language:

The languages of the variant, specified as an Internet
standard language tag from RFC 1766 (e.g.,
en for English, kr for Korean,
etc.).

Content-Encoding:

If the file is compressed, or otherwise encoded, rather
than containing the actual raw data, this says how that was
done. Apache only recognizes encodings that are defined by an
AddEncoding
directive. This normally includes the encodings
x-compress for compress'd files, and
x-gzip for gzip'd files. The x-
prefix is ignored for encoding comparisons.

Content-Length:

The size of the file. Specifying content lengths in the
type-map allows the server to compare file sizes without
checking the actual files.

Description:

A human-readable textual description of the variant. If
Apache cannot find any appropriate variant to return, it will
return an error response which lists all available variants
instead. Such a variant list will include the human-readable
variant descriptions.

Multiviews

MultiViews is a per-directory option, meaning
it can be set with an Options directive within a
<Directory>, <Location>
or <Files> section in
access.conf, or (if AllowOverride is
properly set) in .htaccess files. Note that
Options All does not set MultiViews;
you have to ask for it by name.

The effect of MultiViews is as follows: if the
server receives a request for /some/dir/foo, if
/some/dir has MultiViews enabled, and
/some/dir/foo does not exist, then the
server reads the directory looking for files named foo.*, and
effectively fakes up a type map which names all those files,
assigning them the same media types and content-encodings it
would have if the client had asked for one of them by name. It
then chooses the best match to the client's requirements.

MultiViews may also apply to searches for the
file named by the DirectoryIndex directive, if the
server is trying to index a directory. If the configuration
files specify

DirectoryIndex index

then the server will arbitrate between index.html
and index.html3 if both are present. If neither
are present, and index.cgi is there, the server
will run it.

If one of the files found when reading the directive is a
CGI script, it's not obvious what should happen. The code gives
that case special treatment --- if the request was a POST, or a
GET with QUERY_ARGS or PATH_INFO, the script is given an
extremely high quality rating, and generally invoked; otherwise
it is given an extremely low quality rating, which generally
causes one of the other views (if any) to be retrieved.

The Negotiation Methods

After Apache has obtained a list of the variants for a given
resource, either from a type-map file or from the filenames in
the directory, it invokes one of two methods to decide on the
'best' variant to return, if any. It is not necessary to know
any of the details of how negotiation actually takes place in
order to use Apache's content negotiation features. However the
rest of this document explains the methods used for those
interested.

There are two negotiation methods:

Server driven negotiation with the Apache
algorithm is used in the normal case. The Apache
algorithm is explained in more detail below. When this
algorithm is used, Apache can sometimes 'fiddle' the quality
factor of a particular dimension to achieve a better result.
The ways Apache can fiddle quality factors is explained in
more detail below.

Transparent content negotiation is used
when the browser specifically requests this through the
mechanism defined in RFC 2295. This negotiation method gives
the browser full control over deciding on the 'best' variant,
the result is therefore dependent on the specific algorithms
used by the browser. As part of the transparent negotiation
process, the browser can ask Apache to run the 'remote
variant selection algorithm' defined in RFC 2296.

Dimensions of Negotiation

Dimension

Notes

Media Type

Browser indicates preferences with the Accept header
field. Each item can have an associated quality factor.
Variant description can also have a quality factor (the
"qs" parameter).

Language

Browser indicates preferences with the Accept-Language
header field. Each item can have a quality factor. Variants
can be associated with none, one or more than one
language.

Encoding

Browser indicates preference with the Accept-Encoding
header field. Each item can have a quality factor.

Charset

Browser indicates preference with the Accept-Charset
header field. Each item can have a quality factor. Variants
can indicate a charset as a parameter of the media
type.

Apache Negotiation Algorithm

Apache can use the following algorithm to select the 'best'
variant (if any) to return to the browser. This algorithm is
not further configurable. It operates as follows:

First, for each dimension of the negotiation, check the
appropriate Accept* header field and assign a
quality to each variant. If the Accept* header for
any dimension implies that this variant is not acceptable,
eliminate it. If no variants remain, go to step 4.

Select the 'best' variant by a process of elimination. Each
of the following tests is applied in order. Any variants
not selected at each test are eliminated. After each test,
if only one variant remains, select it as the best match
and proceed to step 3. If more than one variant remains,
move on to the next test.

Multiply the quality factor from the Accept header
with the quality-of-source factor for this variant's
media type, and select the variants with the highest
value.

Select the variants with the highest language quality
factor.

Select the variants with the best language match,
using either the order of languages in the
Accept-Language header (if present), or else the order of
languages in the LanguagePriority directive
(if present).

Select the variants with the highest 'level' media
parameter (used to give the version of text/html media
types).

Select variants with the best charset media
parameters, as given on the Accept-Charset header line.
Charset ISO-8859-1 is acceptable unless explicitly
excluded. Variants with a text/* media type
but not explicitly associated with a particular charset
are assumed to be in ISO-8859-1.

Select those variants which have associated charset
media parameters that are not ISO-8859-1. If
there are no such variants, select all variants
instead.

Select the variants with the best encoding. If there
are variants with an encoding that is acceptable to the
user-agent, select only these variants. Otherwise if
there is a mix of encoded and non-encoded variants,
select only the unencoded variants. If either all
variants are encoded or all variants are not encoded,
select all variants.

Select the variants with the smallest content
length.

Select the first variant of those remaining. This
will be either the first listed in the type-map file, or
when variants are read from the directory, the one whose
file name comes first when sorted using ASCII code
order.

The algorithm has now selected one 'best' variant, so
return it as the response. The HTTP response header Vary is
set to indicate the dimensions of negotiation (browsers and
caches can use this information when caching the resource).
End.

To get here means no variant was selected (because none
are acceptable to the browser). Return a 406 status (meaning
"No acceptable representation") with a response body
consisting of an HTML document listing the available
variants. Also set the HTTP Vary header to indicate the
dimensions of variance.

You should be aware that the error message returned by Apache is
neccessarily rather terse and might confuse some users (even though it
lists the available alternatives). If you want to avoid users seeing this
error page, you should organize your documents such that a document in a
default language (or with a default encoding etc.) is always returned if a
document is not available in any of the languages, encodings etc. the
browser asked for.

In particular, if you want a document in a default language to
be returned if a document is not available in any of the languages
a browser asked for, you should create a document with no language
attribute set. See Variants with no
Language below for details.

Apache sometimes changes the quality values from what would
be expected by a strict interpretation of the Apache
negotiation algorithm above. This is to get a better result
from the algorithm for browsers which do not send full or
accurate information. Some of the most popular browsers send
Accept header information which would otherwise result in the
selection of the wrong variant in many cases. If a browser
sends full and correct information these fiddles will not be
applied.

Media Types and Wildcards

The Accept: request header indicates preferences for media
types. It can also include 'wildcard' media types, such as
"image/*" or "*/*" where the * matches any string. So a request
including:

Accept: image/*, */*

would indicate that any type starting "image/" is acceptable,
as is any other type (so the first "image/*" is redundant).
Some browsers routinely send wildcards in addition to explicit
types they can handle. For example:

Accept: text/html, text/plain, image/gif, image/jpeg, */*

The intention of this is to indicate that the explicitly listed
types are preferred, but if a different representation is
available, that is ok too. However under the basic algorithm,
as given above, the */* wildcard has exactly equal preference
to all the other types, so they are not being preferred. The
browser should really have sent a request with a lower quality
(preference) value for *.*, such as:

Accept: text/html, text/plain, image/gif, image/jpeg, */*; q=0.01

The explicit types have no quality factor, so they default to a
preference of 1.0 (the highest). The wildcard */* is given a
low preference of 0.01, so other types will only be returned if
no variant matches an explicitly listed type.

If the Accept: header contains no q factors at all,
Apache sets the q value of "*/*", if present, to 0.01 to
emulate the desired behavior. It also sets the q value of
wildcards of the format "type/*" to 0.02 (so these are
preferred over matches against "*/*". If any media type on the
Accept: header contains a q factor, these special values are
not applied, so requests from browsers which send the
correct information to start with work as expected.

If some of the variants for a particular resource have a
language attribute, and some do not, those variants with no
language are given a very low language quality factor of
0.001.

The reason for setting this language quality factor for variant
with no language to a very low value is to allow for a default
variant which can be supplied if none of the other variants match
the browser's language preferences. This allows you to avoid users
seeing a "406" error page if their browser is set to only accept
languages which you do not offer for the ressource that was
requested.

For example, consider the situation with Multiviews enabled and
three variants:

foo.en.html, language en

foo.fr.html, language en

foo.html, no language

The meaning of a variant with no language is that it is always
acceptable to the browser. If the request is for foo
and the Accept-Language header includes either en or fr (or both)
one of foo.en.html or foo.fr.html will be returned. If the browser
does not list either en or fr as acceptable, foo.html will be
returned instead. If the client requests foo.html
instead, then no negotation will occur since the exact match
will be returned. To avoid this problem, it is sometimes helpful
to name the "no language" variant foo.html.html to assure
that Multiviews and language negotiation will come into play.

Extensions to Transparent Content Negotiation

Apache extends the transparent content negotiation protocol
(RFC 2295) as follows. A new {encoding ..} element
is used in variant lists to label variants which are available
with a specific content-encoding only. The implementation of
the RVSA/1.0 algorithm (RFC 2296) is extended to recognize
encoded variants in the list, and to use them as candidate
variants whenever their encodings are acceptable according to
the Accept-Encoding request header. The RVSA/1.0 implementation
does not round computed quality factors to 5 decimal places
before choosing the best variant.

Note on hyperlinks and naming conventions

If you are using language negotiation you can choose between
different naming conventions, because files can have more than
one extension, and the order of the extensions is normally
irrelevant (see mod_mime
documentation for details).

A typical file has a MIME-type extension (e.g.,
html), maybe an encoding extension (e.g.,
gz), and of course a language extension
(e.g., en) when we have different
language variants of this file.

Examples:

foo.en.html

foo.html.en

foo.en.html.gz

Here some more examples of filenames together with valid and
invalid hyperlinks:

Filename

Valid hyperlink

Invalid hyperlink

foo.html.en

foo
foo.html

-

foo.en.html

foo

foo.html

foo.html.en.gz

foo
foo.html

foo.gz
foo.html.gz

foo.en.html.gz

foo

foo.html
foo.html.gz
foo.gz

foo.gz.html.en

foo
foo.gz
foo.gz.html

foo.html

foo.html.gz.en

foo
foo.html
foo.html.gz

foo.gz

Looking at the table above you will notice that it is always
possible to use the name without any extensions in an hyperlink
(e.g., foo). The advantage is that you
can hide the actual type of a document rsp. file and can change
it later, e.g., from html to
shtml or cgi without changing any
hyperlink references.

If you want to continue to use a MIME-type in your
hyperlinks (e.g.foo.html) the language
extension (including an encoding extension if there is one)
must be on the right hand side of the MIME-type extension
(e.g., foo.html.en).

Note on Caching

When a cache stores a representation, it associates it with
the request URL. The next time that URL is requested, the cache
can use the stored representation. But, if the resource is
negotiable at the server, this might result in only the first
requested variant being cached and subsequent cache hits might
return the wrong response. To prevent this, Apache normally
marks all responses that are returned after content negotiation
as non-cacheable by HTTP/1.0 clients. Apache also supports the
HTTP/1.1 protocol features to allow caching of negotiated
responses.

For requests which come from a HTTP/1.0 compliant client
(either a browser or a cache), the directive
CacheNegotiatedDocs can be used to allow caching of
responses which were subject to negotiation. This directive can
be given in the server config or virtual host, and takes no
arguments. It has no effect on requests from HTTP/1.1 clients.